The present invention relates generally to co-injection molding systems and, in particular to co-injection nozzle assembly with reduced heat transfer between the co-injection molding products.
Injection molds typically comprise stationary and moving mold halves (i.e., the core side and the cavity side) which are closed and clamped together to form a mold cavity therebetween for shaping articles from thermoplastic compositions. The thermoplastic is heated into a molten condition and is injected under pressure through a nozzle and into the mold cavity by means of a screw ram. Injection pressures of 2,000 to 10,000 psi are common at the gate locations. The plastic is allowed to cool to sufficiently harden the thermoplastic whereupon the mold is opened and the hardened articles are removed.
Hot runner manifold systems are well known and are used to convey the synthetic-material melt, plastified by heating, into a plurality of individual sprue bushings positioned in mold plates of the injection-molding system. The manifolds keep the plastic material in a fluid condition while the material flows from the injection machine nozzle through the sprue bushings and into the mold cavity. Hot runner manifold systems provide for use of a plurality of sprue bushings and multi-cavity molds and thus allow increased manufacture of more products. See, for example, U.S. Pat. No. 5,429,493.
Co-injection molding of materials is well known. A co-injection molding apparatus injects two different plastic materials, typically an inner core material and an outer shell material, into a single mold cavity. A co-injection manifold receives material from two different injection machines and combines the two materials into a single stream which flows into a mold or die.
Co-injection processes create a product which is less expensive. A smaller amount of the more expensive outer plastic material is used for the final part, since it only is used for the outer surface or shell of the product. The second material which is co-extruded is typically a less expensive plastic material and forms the inner non-visible core of the product. A co-injection manifold and process are shown, for example, in U.S. Pat. No. 4,376,625.
The core material and the shell material utilized in a co-injection apparatus often have different melting points. Prior art hot runner systems exhibit a great deal of heat transfer between the melt streams, disadvantageously reducing the efficiency of the co-injection molding process.
It is desirable, therefore, to provide an improved co-injection nozzle assembly that provides reduced heat transfer between the materials of the co-injection molding process.